I don't think JavaScript itself is the problem here. Electron is slow to load and uses a lot of memory because it's loading an entire multiprocess Chromium runt

I don't think JavaScript itself is the problem here. Electron is slow to load and uses a lot of memory because it's loading an entire multiprocess Chromium runtime with all of the Web platform implemented, optimized for speed and security rather than memory usage. Microsoft are using React Native here (aside from the Win32 build), which is going to be a lot more lightweight because it relies on the native platform for the heavy lifting.